The staggering costs of the security operation are revealed as one of the protesters, 79-year-old Margaretta D'Arcy, continues to serve her three-month jail sentence after refusing to sign a bond to keep the peace if she was allowed out of prison on temporary release.

Figures compiled by the Department of Justice show that the cost to gardai of providing security for the Shannon flights amounted to €862,574 in 2012 and €758,152 last year.

This comprised salaries and other pay and non-pay related expenditure, the department said.

The Department of Defence pointed out that gardai had the primary responsibility for law and order, including the protection of the internal security of the State.

Among the roles assigned to the Defence Forces was providing aid to gardai when requested.

Since February 5, 2003, gardai had sought support from the Defence Forces and their costs for 2012 and 2013 amounted to €275,008 and €221,693, respectively.

The combined expenditure incurred by gardai and military over the two years amounted to 2,137,427.

PROTECTION

The security duties include not only policing regular protests close to Shannon Airport but also providing protection for every arriving US plane.

Sometimes, this would involve up to six flights a day, officials pointed out.

A senior garda officer told the Irish Independent that the Shannon duties placed huge strains on local operational budgets, not only in terms of expenditure but also in the deployment of resources.

He said those resources could otherwise be put to use in preventing and detecting crime.

There have been several attempts to attack or interfere with flights and on one occasion a US plane was damaged by protesters.

Ms D'Arcy was given a three-month suspended jail term at Ennis District Court in December when she appeared in court on charges related to an unauthorised incursion on Shannon airfield in October 2012.

The sentence was suspended on condition that she enter into a bond to uphold the law for two years and stay out of the Shannon airport zone.

After she refused to sign the undertaking, she was arrested and taken initially to Limerick Prison to serve her sentence, sparking off protests by action groups. She was considered for temporary release shortly after her committal but would not sign the release bonds that meant she would have to stay away from the unauthorised section of Shannon Airport.

Ms D'Arcy was later transferred to Mountjoy Prison. Her visitors have included Sabina Higgins, wife of President Michael D Higgins, and Sinn Fein president Gerry Adams.